FIELD OF THE INVENTION
The invention relates to a magnetic marker strip that generates a signal inside an interrogation zone in which there is present a periodically varying magnetic field with a predetermined fundamental frequency. The signal generated by the marker strip is picked up by a scanning device and, if a higher-order harmonic of the fundamental frequency is detected as being present in the signal, a display is generated.
The magnetic marker comprises a signal strip made from ferromagnetic material with a low coercive field strength onto which there is applied ferromagnetic material whose coercive field strength is distinctly higher than that of the material of the signal strip. Such magnetic marker strips are disclosed, for example, in U.S. Pat. No. 4,222,517 (see German published application DE 30 26 482 A1) and in U.S. Pat. No. 4,553,136 (see European patent EP 0 121 649 B1).
The German publication DE 30 26 482 A1and its progeny, U.S. Pat. No. 4,222,517, are herewith expressly incorporated by reference. There is disclosed a marker strip of the type mentioned above with a signal strip that is relatively long as compared with its width and that emits harmonic-containing signals in a first, unmagnetized state as a consequence of the magnetic field in the interrogation region and emits no harmonic-containing signal in a second state in this magnetic field. The ferromagnetic material having the higher coercive field strength is arranged there in the form of a plurality of deactivating elements at a spacing from one another on the signal strip. The width of the deactivating elements is essentially equal to the width of the signal strip. The deactivating elements switch the signal strip into the first state when they are in a first, unmagnetized state, and they switch the signal strip into the second state when they are in a second, magnetized state.
The signal strips described there are typically produced from crystalline, highly permeable nickel-iron alloys with a high nickel content.
The alloys described in the prior art disclosure are disadvantageous in that they are very sensitive to mechanical deformations such as bending or twisting with reference to their magnetic properties. The sensitivity goes so far that a single instance of bending the signal strip to and fro is enough to cause a complete breakdown of its functional effectiveness.
The use of the magnetic marker strips described in U.S. Pat. No. 4,222,517 and German publication DE 30 26 482 A1is therefore limited to application in providing security for books as goods in libraries. In that case, sensitivity to mechanical deformations play a greatly subordinate role, since mechanical deformation of the signal strips is prevented to the greatest possible extent by the stiffness of the books.
Amorphous, ferromagnetic alloys are proposed in European patent EP 0 017 801 B1 (see U.S. Pat. Nos. 4,298,862, reissued as RE32,427, and 4,484,184, reissued as RE32,428) and in European Patent EP 0 121 649 B1 (see U.S. Pat. No. 4,553,136) as substantially more suitable materials for the low-coercive signal strips in magnetic marker strips. After being bent to and fro, amorphous, ferromagnetic alloys do not change their magnetic properties to the extent of crystalline, ferromagnetic alloys. As a result, the mechanical stress during production of the magnetic marker strips or during their fastening on the item to be secured does not cause an impairment of their functional effectiveness.
U.S. Pat. No. 4,553,136 (EP 0 121 649 B1) proposes for the use of amorphous, ferromagnetic alloys as signal strips for magnetic marker strips selected alloys which have a saturation magnetostriction .lambda..sub.s which is as low as possible and renders the signal independent of internal and external stress states of the signal strip.
It is set forth as a particular advantage in U.S. Pat. No. 4,552,136 and European Patent EP 0 121 649 B1 that the selected alloys taught there already have a B-H loop, which is rectangular, in the manufactured state, that is to say therefore directly after being cast using rapid solidification technology. The shape of the magnetic hysteresis (B-H loop) of the ferromagnetic material is of very great importance for generating a high harmonic. If a metal object is magnetized by introducing a magnetic field into it, a certain magnetization remains after switching off the magnetic field. The remanence of the magnetization of ferromagnetic materials with respect to the field strength is a measurable variable which can be detected using a curvilinear representation which is generally denoted as a B-H loop. The alloys taught there already have the required rectangular B-H loop in the manufactured state without the need for heat treatment. According to the disclosure in U.S. Pat. No. 4,552,136 and European Patent EP 0 121 649 B1, heat treatment is even disadvantageous, since it tends to result in embrittlement of the amorphous, ferromagnetic alloy. The use of heat treatment is therefore described in U.S. Pat. No. 4,552,136 and European Patent EP 0 121 649 B1 only in conjunction with the production of a partially crystalline or crystalline state for better processability.
It is increasingly the case that goods are no longer being provided by the retail trade with magnetic marker strips, but are already being processed with a magnetic marker strip at the manufacturing stage. This is referred to as source tagging. The reliability to deactivate the magnetic marker strips and, at the same time, economic fabrication are rendered urgent requirements by this development, since now very many goods with magnetic marker strips come about independently of whether an individual retail trader is using a corresponding goods security system or not.
Magnetic marker strips currently available use signal strips made from amorphous, ferromagnetic alloys in typical widths of between 0.7 mm and 2.5 mm in lengths of between 30 mm and 90 mm. For the purpose of deactivation, there are applied to these signal strips a ferromagnetic material whose coercive field strength is distinctly higher than that of the material of the signal strip. In this case, these more highly coercive alloys have coercive field strengths of between 15 A/cm and 100 A/cm. As a rule, these more highly coercive strips are between 3 and 15 mm long and are designed to be 2 to 4 mm wider than the signal strips so that they can be fastened.
These deactivation elements are cut to length individually from a feed roll during the production process. As a rule, they are then fastened via adhesive films which also fix the continuous signal strips of the magnetic marker strip.
By comparison with the method of production described in U.S. Pat. No. 4,222,517 and German patent application DE 30 26 482 A1, and illustrated in FIG. 1 thereof, these methods of production have the disadvantage that the materials used run in each case as narrow tapes into the production process, and the deactivation elements must be cut to length in a process step which has to run at a very high cycling speed for economic reasons.
In the method described in U.S. Pat. No. 4,222,517 and German patent application DE 30 26 482 A1, the deactivation elements are fixed as continuous individual, narrow strips on a wide tape of the signal strip, and the finished magnetic marker strip is finally cut to length. The advantage of this method resides in the economizing use of wide tapes for the signal strip, accompanied by the use of a single process of cutting to length per magnetic marker strip instead of the multiple process steps including fastening in the case of the conventional production, described above, of magnetic marker strips with signal strips of amorphous, ferromagnetic alloys.
An attempt was therefore made also to implement the cost-effective production method, taught in U.S. Pat. No. 4,222,517 and German patent application DE 30 26 482 A1, for magnetic marker strips with signal strips made from crystalline nickel-iron alloys with the amorphous, ferromagnetic alloys taught in U.S. Pat. No. 4,552,136 and European Patent EP 0 121 649 B1. Surprisingly, however, it has emerged that the production method taught there cannot be carried out using the amorphous, ferromagnetic selected alloys taught in U.S. Pat. No. 4,552,136 and European Patent EP 0 121 649 B1.
In a first experiment, a wide tape made from an amorphous, ferromagnetic alloy of the composition of Co.sub.58 Fe.sub.5.5 Ni.sub.13 Si.sub.14.5 B.sub.9 was produced by means of rapid solidification technology with a tape width of 54 mm and a mean thickness of 25 .mu.m. The saturation magnetostriction .lambda..sub.s was -0.5 ppm. The saturation induction B.sub.s of the cast tape was 0.7 Tesla. The tape produced also had a rectangular B-H loop with a remanence ratio (synonymous with the "rectangularity") of approximately 85%.
A signal strip with a width of 2 mm was then cut to length from this cast wide tape transverse to the longitudinal axis of the cast wide tape and its harmonics were measured. For this purpose, the signal strip was excited using an alternating magnetic field with an amplitude of 1 A/cm and a frequency of 1 kHz. The signal strip was orientated in this case parallel to the terrestrial magnetic field, which corresponds to a constant field premagnetization of approximately 0.2 A/cm. The variation in induction caused by the alternating field was measured in an air-compensated pickup coil surrounding the center of the signal strip and having 100 turns, use being made of the voltage induced there. The induced voltage was then decomposed by means of a spectral analyzer into its constituent frequencies, that is to say the harmonic analysis was carried out.
Although the material produced exhibited all the criteria taught in U.S. Pat. No. 4,552,136 and European Patent EP 0 121 649 B1, it was, surprisingly, not possible to detect in the induced voltage a harmonic signal, that is to say a harmonic component, which lay above the usual noise level.
In a second experiment, a cast wide tape having the same alloy composition as above was subjected to heat treatment. For this purpose, an approximately 2 kg heavy tape coil was heat- treated for approximately two hours at a temperature of 230.degree. C. During the heat treatment, a constant magnetic field was additionally applied in the circumferential direction of the tape coil, that is to say parallel, therefore, to the casting direction of the wide tape ("longitudinal field treatment"). The strength of the constant magnetic field was set such that the wide tape was ferromagnetically saturated in the direction of the applied constant magnetic field. The field strength was 10 A/cm in this case. It was possible by means of this treatment to improve the "rectangularity" of the B-H loop of the amorphous, ferromagnetic alloy to virtually 100%, with the result that all the criteria required by U.S. Pat. No. 4,552,136 and European Patent EP 0 121 649 B1 were optimally fulfilled.
A signal strip was, in turn, cut to length from the wide tape, heat-treated in such a way, in a fashion transverse to the longitudinal axis of the wide tape, and its harmonics were measured as in the first experiment. Although the amorphous, ferromagnetic alloy now exhibited a virtually perfectly rectangular B-H loop, no harmonic signal of any kind could be detected. The spectral analysis indicated no harmonic components which lay above the usual noise level. Further experiments were set up for a whole range of various alloy compositions. The results are summarized below in Table 1.
Table 1: Exemplary Embodiments According to the Invention
Harmonic response J.sub.s .vertline..lambda..sub.s.vertline. In produced After longitudinal After transverse Composition (at %) (T) (ppm) state field treatment field treatment Co.sub.58 Fe.sub.5.5 Ni.sub.13 Si.sub.14.5 B.sub.9 0.70 &lt;1 NO NO YES Co.sub.52 Fe.sub.5.5 Ni.sub.18 Si.sub.15.5 B.sub.9 0.59 &lt;1 NO NO YES Co.sub.43.3 Fe.sub.6.7 Ni.sub.28 Si.sub.13 B.sub.9 0.58 &lt;1 NO NO YES Co.sub.67.3 Fe.sub.3.7 Mo.sub.1.5 Si.sub.16.5 B.sub.11 0.55 &lt;1 Very slight NO YES Co.sub.71.8 Fe.sub.1 Mn.sub.4 Mo.sub.1 Si.sub.13.2 B.sub.9 0.82 &lt;1 NO NO YES Co.sub.58.5 Fe.sub.5.5 Mn.sub.1 Ni.sub.15 Si.sub.4 B.sub.16.5 0.90 &lt;1 NO NO YES Co.sub.74.5 Fe.sub.15 Mn.sub.4 Si.sub.11 B.sub.9 1.00 &lt;1 NO NO YES Co.sub.31 Fe.sub.6.5 Ni.sub.40.5 Si.sub.13 B.sub.9 0.41 &lt;1 Very slight NO YES
It was possible to confirm the finding for all alloy compositions that amorphous ferromagnetic alloy tapes such as are taught in U.S. Pat. No. 4,552,136 and European Patent EP 0 121 649 B1 cannot be processed to form magnetic marker strips using the production method taught in U.S. Pat. No. 4,222,517 and German patent application DE 30 26 482 A1.